In one such conventional double twisting spindle (German OS No. 20 35 025 which corresponds to U.S. Pat. No. 3,731,478), the compressed-air supply device consists of a hollow conduit which is movable parallel to the spindle axis, the upper free end of which conduit carries a seal and the lower end of which is connected to a diaphragm arranged in a housing. A valve initially tensioned by a spring is provided in the diaphragm. The housing is connected to a compressed-air source through a valve which is operated by a hand lever. When the hand lever is operated, the spindle rotor is stopped by a suitable braking mechanism which is not disclosed in detail. Upon further operation of the hand lever, the valve is opened and compressed air flows into the housing. The hollow conduit is urged by the compressed air against the lower end of the spindle rotor. When the compressed air in the housing exceeds a certain pressure, the check-valve opens and the compressed air flows into the connecting channel of the spindle rotor.
This arrangement is relatively expensive, because a movable hollow conduit, a diaphragm and a housing must be provided for every spindle. Furthermore, a second compressed-air cylinder for operating the spindle brake probably must also be provided.
A basic purpose of the invention is therefore to provide a double twisting spindle having a pneumatically operated threading device of the abovementioned type which is simpler in design and less expensive to manufacture than known devices.